1. Field of the Invention
This invention relates to a process for effectively performing localized, selective annealing of metal strips. More particularly, it concerns a process for selective annealing metal strips with exceptional width accuracy and a minimum of heat-affected area.
According to the present process, metals to be machined or worked to high degrees, such as spring copper alloys typified by phosphor bronze and poorly workable nickel, nickel alloys, and stainless steels, can be selectively annealed in an efficient way to soften only certain requisite portions prior to the working.
Prior Art
Metallic materials, especially spring materials, by nature rarely combine good formability with adequate springiness. Strong spring materials are difficult to form, and this has been a major limitation to the choice of configurations to which the materials are to be formed, such as of terminals, connectors, and switches.
In view of the above, it has been in practice, in fabricating strong spring materials into terminals and the like, to use special care in designing the shapes lest the materials be subject to stringent forming conditions. However, the rapid development of the electronic industry in recent years has intensified the demand for miniaturization of the terminals and other components to such an extent that there is little room now left for the consideration of shapes when the components are to be made of strong spring materials. As an attempt to solve this problem, selectively annealed materials are used in some sectors of the industry. The treatment involves annealing certain portions of metal strips to be subsequently worked under stringent conditions, such as localized drawing or twisting, or tightly closed bending. Selective annealing usually consists in partially heating a metal piece to anneal it in stripe fashion by the use of combustible gas flame, plasma arc, electron beam or the like as the heat source. However, the gas flame cannot effectively achieve the selective annealing, because it is low in energy density and difficult to attain heat concentration due to its waver. It seldom produces a selectively annealed portion with good width accuracy, homogeneity, and with only a limited heat-affected zone. The plasma arc as a heat source does not suit the purposes of the invention, either, despite its high energy density, since it calls for complex control for the stabilization of the arc. The electron beam requires a high vacuum for its functioning as a heat source. For employment in air, it would need a forbiddingly large power output. For these reasons modern practice favors the use of other heat sources, for example, in frared lamps, high-luminance light sources such as iodine and other halogen lamps, and laser. Advantages of laser and high-luminance light sources include high energy density, eminent stability, and ease of electrical output control.
Nevertheless, these new heat sources require long periods of service in achieving selective annealing of strips of metals, especially nonferrous metals and their alloys, and copper and copper-base alloys in particular. Since the rays of light from these sources are partly reflected by the metal strip surface, sufficient energy density is not attained on the surface for rapid annealing. The prolonged heating combines with the thermal conductivity of the copper alloy, for example, to disperse the heat to the surface portion of the work surrounding the objective area of selective annealing, rendering it impossible to perform selective annealing effectively.
The conventional approaches described above are not satisfactory, above all, where weight is placed on width accuracy or where a selectively annealed portion with a minimum of the heat-affected zone is to be obtained. Even if the rays of light from a given source were brought to focus, for example through a plane, ellipsoidal, or parabolic mirror or a condenser, or through a plurality of such mirrors or lenses, upon a work portion to be selectively annealed, they would not still produce an adequate energy density on the surface of the metal strip. Thus, heating for a great length of time is required for selective annealing. The extended heating and the high thermal conductivity of the copper alloy cause the dispersion of heat from the specific portion to be selectively annealed. Consequently, it is impossible to obtain a selectively annealed portion with excellent width accuracy and a minimum of the heat-affected zone. Furthermore, focusing the entire rays of light from such a source upon the intended portion for selective annealing is impractical; it necessarily heats the work area around the portion being selectively annealed. This results in poor dimensional accuracy of the selectively annealed portion. In order to overcome this difficulty, it has been tried to provide a diaphragm to eliminate the scatter of light by which the work surface area surrounding the portion to be selectively annealed would be undesirably heated. Alternatively, the use of a mask has been proposed to expose only the portion to be selectively annealed to the light source and prevent unwanted heating of the surrounding area. However, the former eliminates part of the light rays intended for heating the portion to be selectively annealed, making prolonged heating inevitable. The latter, or masking, involves difficulties in exactly mounting the mask in position to shield the portion other than that for selective annealing. Therefore, no selectively annealed portion with high width accuracy or a limited heat-affected zone can be obtained.